1. Field of the Invention
This invention relates to a sintered ceramic article formed mainly of alumina and characterized by exhibiting both high strength and high toughness at normal room temperature and elevated temperatures (1200.degree. C.) and a method for the production of the sintered ceramic article.
2. Description of the Prior Art
Alumina has a high melting point, is thermally stabile, and excels in resistance to abrasion, insulation property, and resistance to corrosion and, on account of these features, has found extensive utility as an industrial material. For example, the high electric insulating property of alumina is utilized in electronic components such as spark plugs, IC substrates, and packages as well as in light-pervious alumina, bioceram and other such highly functional sintered articles and single-crystal materials. Practical application of alumina as a functional material has thus been extended into a wide range of fields. Application of alumina as a structural material, however, has been slower. This can be traced to the low toughness of alumina, which is in the range of 3 to 4 MPam.sup.1/2. In terms of resistance to oxidation and corrosion, however, alumina is fundamentally superior to such non-oxide ceramic materials as silicon nitride and silicon carbide which are the main ceramic structural materials in use today. It is therefore regarded as a potentially excellent structural material. Moreover, alumina can be fired in the open air, whereas the non-oxide ceramic materials are required to be fired nitrogen, argon or other such non-oxidizing gas atmosphere. A plant for the production of sintered alumina articles is therefore not very expensive to construct. Further, it can be operated at a low running cost.
Various methods have been studied for improving the toughness of alumina. These methods can be roughly divided into the four types: (1) dispersion of ZrO.sub.2, (2) dispersion of platelike particles or whiskers, (3) growth of platelike alumina particles by the addition of a liquid phase forming auxiliary, and (4) the growth of platelike particles as La.sub.2 O.sub.3.llAl.sub.2 O.sub.3 (hereinafter referred to briefly as "LaAl.sub.11 O.sub.18 ") in a coexistent relationship with alumina. The method of (1) consists in utilizing the stress-induced modification of zirconia. It is reported that the alumina produced by this method possesses a high fracture toughness in the range of from 5 to 8 MPam.sup.1/2. Since the mechanism for increasing toughness utilizes the phase modification of zirconia, high toughness can be obtained by this mechanism only at temperatures not exceeding the temperature of the monoclinic system-tetragonal system modification of zirconia (400.degree. to 800.degree. C.).
The method of (2) consists in dispersing in alumina from 10 to 30% by volume, based on alumina, of whiskers or platelike particles of a non-oxide material with form anisotropy such as silicon carbide or silicon nitride. The toughness of alumina is enhanced by causing the dispersed whiskers or platelike particles to produce a bridging or a fiber extracting effect along edges of cracks produced by grain boundary breaking. As reported in J. Am. Ceram. Soc., 67 [12], C267-C269 (1984), this method produces a sintered article with a high fracture toughness on the order of 8.7 MPam.sup.1/2. On the other hand, however, the strength of the sintered article is reduced because the added platelike particles or whiskers function as faults. Besides, since the method requires a large amount of these platelike particles or whiskers to be dispersed, namely as much as 10 to 30% by volume based on alumina, the reinforced alumina must be fired by hot pressing or hot isotactic pressing (HIP). This method is therefore very expensive and is hard to apply for obtaining a sintered article of complicated shape. Further, since a non-oxide ceramic material such as silicon carbide or silicon nitride is used as a dispersing material, it is likely that the reinforced alumina will deteriorate and lose its fortified toughness by oxidation at elevated temperatures and that the product of this method will not endure protracted use either at elevated temperatures or in a corrosive environment. The method of (3), as reported in Mater. Sci., 28, 5953-56 (1993), for example, consists in using silica-calcia, silica-magnesia or other such firing material which reacts with alumina in the firing temperature range and forms a liquid phase, thereby enabling the alumina particles to grow during firing into plates of high anisotropy and consequently increasing the toughness by a mechanism similar to that obtained by the addition of platelike particles. This method, however, has the disadvantage that the sintered article experiences a marked decrease in strength at high temperatures because a low-melting-point vitreous phase persists in the sinter. The method of (4) consists in causing platelike hexaaluminate particles (such as LaAl.sub.11 O.sub.18 and LaMgAl.sub.11 O.sub.18) to grow in an alumina matrix during sintering. The product of this method suffers only a slight decline of mechanical properties because no low-melting-point vitreous phase persists in the sinter. As reported in J. .Am. Ceram. Soc., 75 [9], 2610-12 (1992), the fracture toughness of this product is about 4 MPam.sup.1/2. Thus, this method gives only a small increase in toughness.
From the foregoing it is obvious that the prior art methods are incapable of producing a sintered alumina article having both high strength and high toughness at temperatures ranging from normal room temperature to elevated temperatures under various circumstances including corrosive environments.
An object of this invention is to provide a sintered article formed mainly of alumina which exhibits high strength and high toughness at temperatures ranging from room temperature to elevated temperatures or in corrosive environments and which enables easy fabrication of products even in complicated shapes by a simple process at a low cost.
Another object of the invention is to provide a method for the production of the sintered article.